This invention relates to a novel curable composition comprising an acrylic copolymer and a specified amino acid.
Acrylic copolymers composed of a major proportion of an acrylic ester and/or methacrylic ester and a minor proportion of another copolymerizable monomer have been well known, and found a wide range of application because of their superior weatherability, heat resistance and oil resistance.
The acrylic copolymers can take various forms such as hard resins, soft resins or rubbery elastomers by changing their ingredients, and are directed to uses suited to given types. For example, a hard resin consisting of methyl methacrylate as a main component and acrylic or methacrylic acid as a minor component is useful as a coating agent for metal containers of canned foods and furniture metals, or as a bonding agent for plywood. A soft resin composed of ethyl acrylate and methyl methacrylate as a major component and an acidic monomer such as itaconic acid, acrylic acid or methacrylic acid as a minor component is useful as a textile finishing agent or a binder for non-woven fabrics. On the other hand, a rubbery elastomer containing ethyl acrylate as a major component and chloroethyl vinyl ether as a minor component, and a rubbery elastomer containing a major proportion of ethyl acrylate or butyl crylate and a minor proportion of glycidyl methacrylate and acrylonitrile are used as an oil seal or shaft seal.
In most cases, the acrylic copolymers are cured in order to improve their properties suitable for given uses, such as strength, wash resistance or solvent resistance, whether they are used in the form of a hard resin, soft resin or rubbery elastomer. The conventional practice for curing acrylic copolymers is to render them curable by using an acidic monomer such as acrylic, methacrylic or itaconic acid, an epoxy-containing monomer such as glycidyl acrylate, a monomer containing an ethyleneimine ring such as iminol methacrylate, a hydroxyl-containing monomer such as hydroxypropyl acrylate, or an amino-containing monomer such as acrylamide as a comonomer copolymerizable with the acrylic or methacrylic ester, and to cure them by heating with the use of suitable curing agents according to the imparted crosslinking reactivities of the acrylic copolymers.
Various curing agents have been suggested for this purpose. For the hard resins and soft resins, a precondensate of urea/formaldehyde and a precondensate of melamine/formaldehyde are most frequently used. However, since toxic amounts of formaldehyde are detected from acrylic copolymers cured with these curing agents, their use has recently been placed under rigorous statutory restrictions from the standpoint of pollution control.
When a self-curable comonomer such as N-methylol acrylamide is used the resulting acrylic copolymers can be cured without using curing agents. Toxic amounts of formaldehyde are also detected from the acrylic copolymers cured by this method, and the use of this self-curing method has also been rigorously restricted.
The rubber elastomeric acrylic copolymers, on the other hand, do not cure by the urea/formaldehyde precondensate or the melamine/formaldehyde precondensate. Most commonly used curing agents for the elastomers are, for examples, triethylene tetramine, hexamethylene diamine carbamate, and mercaptoimidazoline. These compounds are highly toxic, and in order to avoid pollution, the use of curing agents having little toxicity has been strongly desired.
With this background, the present inventor found novel curing agents for acrylic copolymers.